OFDM air interfaces will become increasingly important e.g. for future evolutions of air interfaces in 3GPP Radio Access Networks, for Wireless Local Area Networks (WLANs) e.g. according to the IEEE 802.11a standard or for a 4th generation air interface.
In OFDM transmission, frequency patterns are allocated to the mobile terminals. Up to now, different cells have different carrier frequencies or time-frequency patterns that are random-like, so that no interference coordination between the cells is necessary or possible.
Given the licensed bandwidth, transmission capacity from network providers e.g. for WEB surfing or video streaming has to be as high as possible for all users to serve as many subscribers as possible. Further the quality of service experienced by the user and the coverage of the service is an important property demanded by the user. So OFDM shall also work at the cell border.
A frequency re-use factor of 1 for the different cells and interference coordination shall be achieved for OFDM transmission in order to increase the utilization of the bandwidth without degradation of the quality of service caused by inter-cell interference.
In cellular systems with a frequency re-use factor of 1 the signal to interference ratio at the cell border approaches the factor 1 or 0 dB, so that no useful transmission from the base station to the mobile terminal can be kept up. Therefore in CDMA systems (CDMA=Code Division Multiple Access) soft handover was introduced using a different code from the neighboring cell in addition to the primary code from the serving cell. For packet transmission using High Speed Downlink Packet Access (HSDPA) no such solution is given reducing the coverage of HSDPA transmission to a fraction of the cell area.
In OFDM transmission, frequency patterns are allocated to a mobile terminal instead of codes in CDMA systems. In OFDM transmission, in contrast to CDMA transmission, interference can be planned and avoided. For OFDM transmission, which does not provide different scrambling codes for the different base stations, the problem at the cell border has to be solved as well. For that purpose and e.g. unsynchronized base stations frequency patterns are allocated to the users and the caused cross-cell interference can be coordinated.
Additionally, using a selective allocation of subcarrier frequencies will exploit the channel capacity by allocating to a user subcarrier frequencies, e.g. a block of adjacent subcarrier frequencies, whose corresponding signals are amplified by its channel transfer function. That means that a frequency selective frequency pattern, which is best suited for each user, shall be allocated to him to run multi-user diversity.
For the coordination of interferences, the frequency patterns have to be the same in neighbor cells while the pilot subgrid in neighbor cells shall be different to allow channel estimation also in the interference region. So these frequency patterns have to be compatible with all possible pilot subgrids, meaning that the number of pilot hits, i.e. stolen subcarrier frequencies by pilots, needs to be independent of the pilot subgrid. Further to this compatibility requirement, a change from a set of frequency diverse to a set of frequency selective frequency patterns must be possible to allow an adaptation to the changing ratio of stationary users to non-stationary users.
The object of the invention is to propose a method for distributing data on an OFDM time-frequency grid for data transmission allowing for coordination of interferences between different cells of the mobile network and frequency scheduling, i.e. adaptive subcarrier frequency allocation to mobile terminals of said mobile network.
This object is achieved by a method for distributing data on an OFDM time-frequency grid for data transmission from and to mobile terminals in a mobile network allowing for coordination of interferences between different cells of the mobile network and adaptive subcarrier frequency allocation to mobile terminals, whereby subcarrier frequencies of the OFDM time-frequency grid are gathered in frequency patterns, wherein                the frequency patterns are constructed to accommodate the use of pilot subgrids,        a set of frequency patterns is changed in a pilot compatible manner between frequency diverse and frequency selective frequency patterns,        and said frequency patterns are allocated to mobile terminals for data transmission,a base transceiver station comprising means for distributing data on an OFDM time-frequency grid for data transmission to mobile terminals in a mobile network allowing for coordination of interferences between different cells of the mobile network and adaptive subcarrier frequency allocation to mobile terminals, whereby subcarrier frequencies of the OFDM time-frequency grid are gathered in frequency patterns, wherein        the frequency patterns are constructed to accommodate the use of pilot subgrids,        a set of frequency patterns is changed in a pilot compatible manner between frequency diverse and frequency selective frequency patterns,        that said frequency patterns are allocated to mobile terminals for data transmission,a base station controller for radio resource management in a mobile network, wherein the base station controller comprises means for allocating the frequency patterns according to claim 1 to the cells of the mobile network and a mobile network comprising mobile terminals, at least one base transceiver station comprising means for distributing data on an OFDM time-frequency grid for data transmission to mobile terminals in a mobile network allowing for coordination of interferences between different cells of the mobile network and adaptive subcarrier frequency allocation to mobile terminals, whereby subcarrier frequencies of the OFDM time-frequency grid are gathered in frequency patterns, wherein        the frequency patterns are constructed to accommodate the use of pilot subgrids,        a set of frequency patterns is changed in a pilot compatible manner between frequency diverse and frequency selective frequency patterns,        that said frequency patterns are allocated to mobile terminals for data transmission        
and at least one base station controller for radio resource management in a mobile network, wherein the base station controller comprises means for allocating the frequency patterns according to claim 1 to the cells of the mobile network.
In the following, the main idea of the invention will be shortly described.
It shall be assumed that N is the total number of subcarrier frequencies in the frequency band. If M is the number of subcarrier frequencies contained in each frequency pattern, J frequency patterns with J·M=N can be constructed. This number J is often a power of 2 to allow efficient signaling of the patterns allocated to a user. Distributing the subcarrier frequencies of each frequency pattern as far apart as possible to achieve maximum frequency diversity results in frequency patterns, whose subcarrier frequencies all lie J subcarriers apart. The placement of all frequency patterns resembles then a simple interleaving arrangement.
These frequency patterns have to be compatible with all pilot subgrids where every p-th subcarrier is a pilot carrier. Ideally, if the number of pilot hits in the different frequency patterns is equally distributed over all frequency patterns, there should be a maximum number of pilot hits MAX per each frequency pattern that can be calculated according to this formula:MAX=ceil(M/p)
If the distance J of the subcarrier frequencies in each frequency pattern and the distance p of the pilot subgrid subcarrier frequencies have common prime factors, the problem occurs that some frequency patterns have a lot of pilot hits while others have none. To achieve a more equal distribution of the pilots over the frequency patterns, in some of the interleaving intervals at least one cyclic shift of the respective part of the numbered frequency patterns is performed rotating the allocation of the subcarrier frequencies to the different frequency patterns inside the interleaving interval. This procedure will be described below in more detail.
The main idea of the invention is now to select M frequency patterns with (again) adjacent subcarrier frequencies. Then, in all M interleaving intervals, the M subcarrier frequencies are lined up and now, by uniting the M subcarrier frequencies of an interleaving interval, a new frequency selective frequency pattern is created. This is done in each interleaving interval. Then, the M frequency patterns with interleaved subcarrier frequencies are replaced by the M new frequency selective frequency patterns, each of them comprising just a single block of M adjacent subcarrier frequencies.
The new frequency patterns are frequency selective by just using a narrow region of the frequency spectrum. Further each new frequency pattern is also compatible with the pilot subgrid of pilot distance p since it is obvious that for any shift not more than ceil(M/p) pilots can fall in the block of width M.
Thus starting from the group of M frequency diverse frequency patterns, a new group of frequency patterns, all compatible with the pilot subgrid, can be formed containing J-M frequency diverse and M frequency selective frequency patterns. Depending on the number M, this transformation can be carried out several times starting at desired frequency positions .
With this procedure, a group of frequency patterns can be constructed that contains as well frequency diverse as frequency selective frequency patterns which are all compatible with the pilot subgrid.
The group of frequency patterns can be adapted to the traffic need, i.e. the distribution of the users on the categories stationary or non-stationary.
For the different cells of the mobile network now a fixed or negotiated power limitation on a subset of the frequency patterns is in place. The subset is different for the different cells. The non-stationary users get now allocated frequency diverse frequency patterns according to the interference sources sensed by the corresponding mobile terminals and exploiting the power limitations on interfering frequency patterns from neighbor cells. The stationary users get allocated frequency pattern also obeying the possible power limitations but they get the frequency selective frequency pattern based on their specific channel transfer function which can be signaled back.
With these frequency patterns, frequency coordination in the cell overlapping region and adaptive subcarrier allocation, i.e. frequency scheduling, is now possible at the same time.
Further developments of the invention can be gathered from the dependent claims and the following description.